reftable/stack: use `size_t` to track stack length

reftable/stack: use `size_t` to track stack length

While the stack length is already stored as `size_t`, we frequently use
`int`s to refer to those stacks throughout the reftable library. Convert
those cases to use `size_t` instead to make things consistent.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>